AN2783 datasheet(21/47 Pages) STMICROELECTRONICS

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AN2783

PoE section

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8.6

AUXI input

The PM8800 reference board accepts auxiliary power sources applied before the hot-swap

MOSFET as low as 18 V (16 V seen at the pin VIN of device).

To do so and change the UVLO levels, the AUXI pin must be pulled up above 2 V with a

current greater than 70 µA.

The AUXI pin can be connected to the auxiliary voltage through a diode. In this case the

current flowing into the pin is internally limited to about 300 µA.

Depending on the output current drawn, the real operative AUXI voltage can be higher than

the above mentioned value, basically due to the DC current limitation which is maximum

input power at minimum applied on AUXI = 16V x 800 mA =12.8 W.

Another limitation on the operative AUXI voltage can be the power transformer, not designed

to work with a wide input voltage range for the maximum output power.

8.7

AUXII input

PM8800 can also accept auxiliary power sources applied after the hot-swap MOSFET as

slow as 12 V (9 V seen at the pins of device). In this case there is no current limitation and

an external circuit is recommended in order to limit the inrush current.

On the PM8800 reference board an active switch is implemented with a P-channel power

MOSFET, capable of limiting the inrush current at startup and with very low ohmic drop

during operation.

AUXII prevalence over PoE can be programmed forcing a current higher than 100

µA in pin

AUXII of PM8800. In this case the PD is always powered from AUXII power source because

the interface circuits and the hot-swap MOSFET are forced in an off state. The pin can be

connected to the auxiliary voltage through a diode. In this configuration the current flowing

into the pin is internally limited to about 250 µA. AUXII can be conveniently used in case of

high-power PDs requiring input power higher than the 12.95 W specified in the IEEE802.3af

standard.

Please note that having the hot-swap MOSFET in an off state means having the IC

substrate in high impedance with GND. It is strongly recommended to move the signature

capacitor of 100 nF from the C1 position to C36. This capacitor is placed between VSS and

GND, implementing a low-impedance circuit at high frequency across the hot-swap

MOSFET, assuring a good HF connection of the IC substrate. The PM8800 reference board

is preset for AUXII prevalence over PoE, having R23 set to 10 k

Ω.

Warning:

In case of AUXII low input voltage sources, the condition

VIN < VCC must be avoided because of possible damage to

the device.

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